In the prior art of measurement of mechanical stress, monitoring of physical movement, and measurement of temperature of physical objects, it is known to place a sensing means along the length of the object to be measured
U.S. Pat. No. 4,654,520 to Griffiths discloses the use of an optical fiber which is securely fastened to a long, continuous structure such as a pipe line. Light is passed into an end of the optical fiber and any physical movement of the structure including deflection, bending, displacement or fracture produces changes in the optical fiber which can be measured by means of reflection of light from the fiber, or by means of light which is passed through the fiber to the other end.
In Griffiths, the detecting means measure and display in the time domain the reflected intensity from various points along the fiber. This technique is based upon changes in reflection (Rayleigh backscattering) and detection of light that is passed completely through the fiber. All of the measurement techniques are dependent upon the ability to measure accurately the light emanating from the optical fiber end.
The Griffith approach falls into the general category of optical time domain reflectometry. A short pulse is launched down the fiber and a sensor detects the light continuously backscattered by the fiber core. The exponential attenuation rate of a fiber of this type is normally known. Stresses applied to the fiber will cause a drop in light intensity at the location of each stress, and the stress amplitude and its location can be determined.
The optical time domain reflectometry approach suffers from the weakness that the backscattered signal is normally small and the signal reflected out of the fiber optic core is small. The measure of stresses in tens of meters of fiber length requires a transmitted optical pulse of a few nanoseconds in length. Such narrow pulses limit the magnitude of the energy that can be put into such a fiber.
Interferometry may also be used with fiber optics to detect stress or changes in the optical fiber. The signal in the sensing fiber is compared to the signal in a reference fiber and any phase shift between the two measured. This phase shift is sensitive to sensing fiber elongation (strain) and to changes in light velocity, as for example affected by the refractive index.
The weaknesses of the interferometer approach are (1) implementation is mechanically very delicate and (2) changes in fiber temperature result in strain changes. Thus, changes in fiber temperature appear as stress changes. The nature of the measurement is not distributive along the fiber, but it is a net, cumulative effect. Multiple stresses along the fiber cannot be resolved either in amplitude or in location.